Because of expectation for realization of an optical network (photonic network) having high flexibility, it is required for a node configuring the network to cope with a large change in the number of wavelengths, which are supposed to be a communication path. Particularly, an optical amplifier which is a constitutional element of a node is required to cope with a large change in allocation and the number of wavelengths to stabilize the amplification characteristic.
The wavelength characteristic of an amplification medium such as an EDFA (Erbium Doped Fiber Amplifier) used in a known optical network system, in which allocation and the number of the wavelengths are assumed not to be largely changed, can be presupposed to depend upon only population inversion by a single band approximation (refer to non-patent document 1). Namely, the wavelength characteristic can be approximated and grasped according to the value of the population inversion rate, with the whole amplification band of the EDFA being one unit.
In concrete, as shown in FIG. 24, a pattern of relative gain coefficients as being the wavelength characteristic over the whole range of the amplification bandwidth (wavelengths from 1500 to 1580 nm of an input signal light in the drawing) of the EDFA can be grasped for each population inversion rate. Accordingly, wavelength flatness of the EDF in C band (Conventional Band) is realized by combining the automatic gain control by which the population inversion is controlled to be constant, and the gain equalizer according to the relative gain coefficient distribution corresponding to the population inversion that is controlled to be constant.
FIG. 25 shows an example of the structure of an optical repeater 100 used in a known optical network system in which wavelength allocation and the number of wavelengths are not largely changed. The optical repeater 100 shown in FIG. 25 is configured by inserting an optical attenuator (VOA: Variable Optical Attenuator) 102 between two EDFA amplifying units 101-1 and 101-2 serially connected.
Each of the EDFA amplifying units 101-1 and 101-2 comprises branching couplers 101a and 101b, an EDFA 101c, photodiodes (PD: Photo Diode) 101d and 101e, and a control circuit 101f. In each of the EDFA amplifying units 101-1 and 101-2, the input/output powers are monitored by the respective photodiodes 101d and 101e, and an optical signal amplified by the EDFA 101c under the automatic gain control by the control circuit 101f is outputted.
As shown in FIG. 26, for example, when the input power of the optical repeater 100 is changed from the first level to the second level, the output power of the optical repeater 100 is made constant by adjusting the quantity of loss in the variable optical attenuator 102 while keeping the gain in each of the EDFA amplifying units 101-1 and 101-2 constant.
If the amplification characteristic of the EDFA 101c in each of the EDFA amplifying units 101-1 and 101-2 is assumed to be an optical network that can be approximated to a single band, the gain wavelength characteristic can be always kept constant by keeping the gain of each of the EDFA 101c constant. Accordingly, it becomes possible to make the gain wavelength characteristic of the optical repeater 100 flat irrespective of the input power, by disposing a gain equalizer whose loss characteristic is appropriately designed in the following stage of the EDFA amplifying units 101-1 and 101-2.
Namely, since it is supposed that the known optical repeater is applied to an optical network in which wavelength allocation and the number of wavelengths are not largely changed, the gain equalizer arranged in the following stage of the EDFA amplifying units 101-1 and 101-2 is designed on the assumption that the wavelength characteristic of an amplifying medium as above is approximated to a single band.
However, in an optical network recently demanded in which wavelength allocation and the number of wavelengths are largely changed, the gain deviation due to an effect of spectral hole burning (SHB: Spectral-Hole Burning) which is a local gain saturation effect in the wavelength region cannot be ignored when the selected wavelengths are arranged to be particularly gathered in a narrow band. Since the effect of this SHB differs according to wavelength allocation supposed in the optical network, it is necessary to analyze the gain deviation caused by SHB according to the wavelength allocation beforehand supposed when the apparatus is designed.
FIG. 27 shows gain deviation characteristic due to SHB of an EDFA. When a gain wavelength characteristic A in the saturated state where a saturation signal at 1540 nm (signal saturating the gain of the EDFA) is compared with a gain wavelength characteristic B in the non-saturated state where no saturation signal is inputted, it can be confirmed that, in the saturated state, the gain in the vicinity of the saturation signal wavelength and 1530 nm is decreased (refer to a gain difference C between the characteristics A and B) to make holes.
This phenomenon occurs due to a local gain saturation phenomenon of a gain medium having inhomogeneous broadening. In the known single band approximation, this local change in gain wavelength characteristic is ignored.
As a model of EDFA in which SHB is considered, there have been reported a model (refer to non-patent document 2) which separately deals with the absorption/emission process and the saturation process between energy levels formed by the inhomogeneous broadening, and a model which adds the quantity of gain fluctuation due to SHB derived from a result obtained by separately measuring the gain wavelength characteristic obtained by means of single band approximation (refer to non-patent document 3).
As techniques relating to the present invention, there are also techniques described in Patent Document 1 and Patent Document 2 shown below.
However, the technique described in Non-Patent Document 2 provides a very complex calculation formula for analyzing the gain deviation, thus has a disadvantage that the process requires a long time. The technique described in Non-Patent Document 3 considers only the neighborhood of the signal wavelength, thus has a disadvantage that the gain fluctuation in the vicinity of 1530 nm cannot be modeled.
As a method of measuring the amplification characteristic of an amplification medium, there is a method (hardware simulation) for measuring the amplification characteristic from an actually formed optical amplifier other than the method of calculating through numerical value calculation described above. However, some sorts of hardware simulation take a long time or require much labor to measure entirely the wide operation conditions of an optical repeater.
In the light of the above problems, an object of the present invention is to provide an apparatus and a method for amplification medium performance simulation, and an optical amplifier, which introduce a simple approximate expression, thereby modeling gain fluctuation in a range other than the neighborhood of the signal wave within a short time.
Non-Patent Document 1: C. R. Giles, et al., “Modeling Erbium Doped Fiber Amplifiers,” IEEE J. of Lightwave Tchnol., pp. 271–283, vol. 9, no. 2, Feb., 1991;
Non-Patent Document 2: E. Desurvire, “ERBIUM-DOPED FIBER AMPLIFIERS Principles and Applications,” John Wiley & Sons, Inc., Chapter 4, 1994;
Non-Patent Document 3: T. Aizawa, et al., “Effect of Spectral-Hole Burning on Multi Channel EDFA Gain Profile,” OFC'99, WG1, 1999;
Patent Document 1: Japanese Patent Application Laid-Open No. 2000-261078; and
Patent Document 2: Japanese Patent Application Laid-Open No. 2000-261079.